This invention relates to a method of manufacturing polygonally shaped, multifaceted rotary scanners by replication and to multifaceted polygonally shaped scanners manufactured by the method.
In particular, this invention relates to a method of manufacturing polygonally shaped, multifaceted rotary scanners of the kind which comprise a plurality of optically flat, reflective surfaces integrally mounted at precise angles to the axis of rotation on a multifaceted polygonal base.
Multifaceted polygonal scanners can be employed in a wide range of applications, typically for the accurate reflection of light or other electromagnetic radiation, such as thermal radiation, from a radiation source for passage along a narrow path of a radiation sensor remote from the radiation source.
The angle at which each radiation reflective facet of the multifaceted polygonal scanner is set is subject to misalignment during manufacture and when the scanner is in use. The angle of each reflective facet is known to be critical because it determines the direction of the reflected radiation and the duration of the scan relative to the angular velocity of the rotating scanner. The relative angles at which the radiation reflective facets are set to each other also must be constant even when the scanner is subjected to large induced stresses due to high speed rotation when the scanner is in use.
From U.S. Pat. No. 4,093,350, it is known to produce high speed, multifaceted polygonal scanners by a replication technique.
In the aforesaid U.S. patent, there is described a process which user a multifaceted polygonal master form, the inner periphery of which comprises a plurality of facet-shaped members. An aluminium preform is positioned concentrically within the master, and an epoxy is centrifugally cast against the aluminum preform in a replication process to provide the multifaceted polygonal scanner. The cast epoxy conforms to the shape of the faceted master.
The epoxy is required in U.S. Pat. No. 4,093,350 to be cast by centrifugal dispersion into a single, circumferential gap, in a one stage operation while the master form is rotated on a vertical axis. The gap is positioned between the inner periphery of the master form and the outer periphery of the preform comprising the facet-shaped members. Dispersion of the epoxy is induced by the centrifugal casting and continues until the gap is filled with a thin film of bubble free liquid epoxy.
It is furthermore stated in their patent, that in prior art replication techniques the production of large optical surfaces has been contemplated so that the application of a thin plastic film to the preform is found to be relatively easy. However, the ease of accessibility of these surfaces unfortunately does not always ensure a bubble-free plastic layer. The thicknesses of these layers may be, for example, approximately 0.005 inches, so that to fill the narrow cavity between the master form and the polygonal preform without introducing of air bubbles is extremely difficult, if not impossible. Thus, such thin films of liquid resin are cast by centrifugal means in the one stage operation.
The bubble-free epoxy film is next cured. The master form, however, then has to be removed without damaging either the preform or the master (which is required for subsequent use). The preform is removed by a pushing action, or as illustrated by FIG. 1 of the aforesaid U.S. patent, by the use of screws which apply pressures at various disengagement points. It is clear that great care must be exercised if damage and distortion are to be avoided, and this can be both time consuming and expensive, particularly when surplus resin becomes firmly set onto variously angled nonpolished surfaces of the master form.
The aforesaid process furthermore does not lend itself to a cheap, simple manufacture of a multifaceted preform, the surfaces of which are accurately produced for concentric positioning within the accurately produced master. Coating the surfaces of a plurality of facets with a single layer of gold may also be difficult to effect and expensive in material particularly if various surfaces are almost inaccessible. On removal by breaking the master away from the preform, the gold layer so produced must separate from the surface of each facet of the master form while remaining adhesively bonded to the hardened epoxy. Good separation obviously becomes increasingly difficult with an increase in the number of facets on the faceted master and with an increase in the changes of the angles of the facets.